This invention relates to the volume production of easy-open can covers or can ends. More particularly the invention is concerned with providing an improved progressive punch press and production technique for making large quantities of sheet metal can covers formed with integral "push-button" type easy-open closures. While the invention will be described in relation to mechanism for practicing the method disclosed in the above-cited application for purposes of illustration, it will be recognized that in various aspects the invention is not thus limited in use, nor necessarily to the making of any particular type of easy-open can end.
Systems for producing can ends have hitherto employed a punch press with sequentially operated stations and carrier means (for instance a linearly moving belt or turret-like guide ring) having pockets for holding and transferring each can end from station-to-station by engagement with the curled outer flange, countersink, or periphery of the respective ends. Where can covers, ends, caps or like articles need not be made with precision, as where there is fairly broad dimensional tolerance on the outer profile of the item, for instance, such approaches have been reasonably acceptable. Typical transfer systems used with conversion presses do not, however, afford locating accuracy not to exceed, say 0.0015 inches in "Total Indicated Reading", and such accuracy is highly desirable in the production of many workpieces such as easy-open covers of the type above indicated.
In transferring the work from station-to-station between upper and lower cooperative forming tools, a spring means has served as the press opens to lift the can end carrier means clear of the dies thus to provide clearance for work advancement to a next station. Conversely, upon press closing of an upper die, usually acting through a countersink guide ring, the can end has been depressed and the curled or flanged can end periphery brought to bear with pressure upon the carrier means. These practices impose constraints that preclude exact self-alignments of the work, or critical portions thereof, at each station with respect to its tooling. It is appreciated that these basic elements of the can end transfer means, whether of the belt or guide ring type, are well accepted in the industry and in general desirably are to be retained.
By way of illustrating a can end production system wherein the apparatus includes end sequencing and, additionally, tear tab processing, reference may be had to U.S. Pat. No. 3,196,817 of July 27, 1965.
It has been discovered that for precision die operations to be performed sequentially in sheet metal, especially those wherein manually openable weakening lines of closures are to be uniformly and repeatedly produced with their sealing integrity consistently intact, stress loads to the point of deformation must be avoided. While lateral registration of the work relative to the cooperative dies when presented by the work transfer means is of some importance, it has been found that it is of at least equal importance to the avoidance of high stress loads at the closure within a cover or can end to attain precise vertical alignment of the cooperating dies with the work at the successive stages. Accordingly, the present invention contemplates the use of work-supporting and indexing mechanism for yieldably acting on the can end outer margins and allowing the can ends to be self-accommodating laterally and heightwise at the different stations. This is to say that, by reason of the "give" and reduction in slidable friction afforded to each workpiece as a whole by a yieldable clamping of the outer margin thereof, its closure portion can be progressively made substantially stress-free, and refined in its exact form, position, and potential operating condition by successive working of the cooperative pairs of upper and lower forming, coining and swaging dies. This procedure contrasts with prior art in which the workpiece is rigidly held causing the dies operable thereon to induce localized stresses often exhibited in a warped product; in fact, the lack of "give" in such prior practice with can ends may induce inconsistency and actual premature separation of the metal edges defining a weakening line.